With the rapid miniaturization of portable electronics, such as cell phones, hand held computing devices, wireless sensors, and remote monitoring applications, the availability of a reliable power source remains an important design issue. Many applications for wireless sensing exist where battery replacement is economically and logistically impractical, laborious, and potentially dangerous. Collecting and storing power from ambient energy harvesting devices remains a desirable option for greatly extending battery life, replenishing depleted batteries, or simply replacing the need for batteries entirely.
Ambient energy harvesting is increasingly gaining acceptance as a potential power source that can be used for numerous commercial and military applications. But a problem for power electronic designers is incorporating energy harvesting methods into their designs given the relatively small voltage levels typically available from energy harvesting device technologies. In order to solve this problem, an ultra-low input voltage power conversion method is needed to convert small amounts of scavenged energy into a usable form of electricity. Such a method would enable new and improved medical devices, sensor systems, and other portable electronic products.
Thermoelectric harvesting devices deliver relatively high currents at sub-transistor threshold voltages when exposed to low temperature gradients. These voltages are on the order of 20 mV, which is much lower than the operational threshold voltages for standard power conversion electronic integrated circuits. What is needed then is a low-power, ultra-low input voltage DC to DC converter for thermoelectric energy harvesting at low temperature gradients.
Experienced power designers have explored using magnetics, such as a transformer controlled by MOSFETs driven from a low voltage oscillator to boost small voltages. For example, Linear Technologies offers an “Ultralow Voltage Step-Up Converter and Power Manager” called the LTC3108. The LTC3108 has an efficiency of about 50%, and the output current is stated to be 3 mA. Both of these specifications are lower than desired for thermoelectric generators that output relatively high current and ultra-low voltage when exposed to a small thermal gradient.